The tacticity of poly(methyl methacrylate) prepared by free-radical polymerization has been determined by analysing the carbonyl region of the 13C n.m.r . spectrum. This analysis demonstrates that the addition of poly(methyl methacrylyl) radical to methyl methacrylate in benzene at 60°C is subject to a significant penpenultimate unit effect. The probability of forming a meso dyad is some 20% less if the preceding dyad in the chain is also meso. The polymerization (60°C, benzene) is described by the following parameters P(m) = 0.202, P(m|m) = 0.159, P(r | m) = 0.212.